1. Field of the Invention
The present invention relates to an X-ray generation detector which detects the X-ray generation of an X-ray generator and outputs an X-ray exposure signal, and also to an X-ray imaging apparatus which detects in the form of an electric signal an X-ray image relating to an intraoral region or the like of a subject so that the X-ray image is displayed on a CRT (cathode ray tube) or the like.
2. Description of the Related Art
Conventionally, in order to obtain an X-ray image of an intraoral region, a film method is widely employed in which a photosensitive recording material such as a silver salt applied film is exposed to an X-ray image and the X-ray image is then subjected to developing and fixing processes.
However, the film method has problems including: 1) a time span of about 2 minutes or longer must be consumed between the X-ray radiography and the observation of the X-ray image; 2) a developing apparatus and processing solutions for conducting the developing and fixing processes are indispensable; 3) the X-ray sensitivity of a silver salt has a limitation and therefore an X-ray dose of a predetermined level is required to obtain a desired image density; and 4) it is impossible to correct an image which has been once fixed.
In order to solve these problems, an X-ray imaging apparatus is proposed in which an X-ray image is converted into an electric signal by using an imager device such as a CCD (charge coupled device) and the X-ray image is then displayed on a CRT (cathode ray tube) or the like. The X-ray imaging apparatus employs a so-called film-less method which does not use a photosensitive recording material such as a silver salt applied film, and has features including: 1) an X-ray image can be observed in real time after the X-ray radiography; 2) a developing apparatus and processing solutions are entirely unnecessary; 3) the X-ray sensitivity characteristic of an imager device is linear and therefore an X-ray dose can be reduced; and 4) a detected X-ray image can be subjected to various image processing, and easily copied or stored.
In the conventional film method, at the time when an X-ray film is positioned in the aral cavity of a patient, the preparation for X-ray exposure is completed so that the X-ray exposure can be done at any time thereafter.
On the other hand, in the X-ray imaging apparatus, a so-called idle reading operation in which charges (charges due to X-ray photon incidence, charges due to thermal excitation, etc.) accumulated in the light receiving portion of the imager device are periodically read out is conducted to reduce the dark current level of the imager device. Consequently, there arises a problem such that, when the X-ray radiography is conducted during this operation, a perfect X-ray image cannot be obtained. Furthermore, in a period when image processor conducts a predetermined image processing of an X-ray image obtained in the previous radiography, or another processing, even when another X-ray exposure is newly conducted on the imager device, the image processor cannot fetch a signal from the imager device, resulting in that the X-ray image formation ends in failure.
Moreover, the X-ray imaging apparatus is configured independently of an X-ray generator, and these two units do not connect with each other. This produces a problem such that the X-ray imaging apparatus cannot know the time when an X-ray radiography started and hence cannot decide the timing of starting the operation of reading an image from the imager device. Accordingly, the operator must separately control the X-ray generator and the X-ray imaging apparatus, thereby making the work of obtaining an X-ray image cumbersome. If the X-ray radiographing and the operation of reading an image from the imager device fail to establish a constant timing relationship between each other, furthermore, the dark current level and the noise level vary in each radiography, thereby producing a problem such that a stable X-ray image cannot be obtained.
In a usual dental Roentgen system, moreover, a film is set in the mouth of a patient, and an X-ray radiographing is then conducted by irradiating the face of the patient with X-rays so that an X-ray image of teeth or the like is recorded on the film in a positional relationship established in a direction along which the operator sees the patient. When the X-ray image recorded on the film is to be observed, such a positional relationship is convenient for the operator because the observation can be conducted as if the operator opposes the patient. Since an image of the right portion of the patient appears in the left portion of the film, however, the patient must observe the image in which the right and left sides are interchanged. Consequently, it is difficult for a patient inexperienced in the observation of an X-ray image to observe such an image.
In a conventional film photography, a mirror image can be obtained by turning over a film, and therefore the above-mentioned problem is not a serious one.
However, a conventional X-ray imaging apparatus wherein an X-ray image is displayed on a CRT or the like is not provided with a function of displaying a mirror image which is obtained by reversing an original image. When a dentist explains to a patient his(her) disease condition, therefore, it is unavoidable that the patient cannot fully understand his(her) disease condition, or the prolonged time for the explanation is required.
In a conventional X-ray imaging method using a silver salt applied film, the sensitivity of the film is fixed. By contrast, in an X-ray imaging apparatus using an imager device, however, the sensitivity is changed depending on the gain of a signal processing circuit. Specifically, when the X-ray dose (=X-ray intensity X irradiation time) is increased, the quantum efficiency of X-rays is decreased, so that a higher definite image is obtained, and when the X-ray dose is decreased, the image quality is lowered but the radiation exposure dose of a patient can be reduced. In this way, the X-ray radiation dose can be arbitrarily selected in consideration of the relationship between the balance of the image quality and the radiation exposure dose. In order to obtain an adequate image irrespective of the X-ray radiation dose, the X-ray radiation dose and the gain of the signal processing circuit must be linked together.
However, a conventional X-ray imaging apparatus is disposed independently of an X-ray generator. When X-ray generation conditions such as the X-ray radiation dose are to be adjusted, therefore, the X-ray generator must be operated, and when image process conditions such as the gain of a signal processing circuit are to be adjusted, the X-ray imaging apparatus must be operated. These make the adjusting operation very troublesome.
Furthermore, the X-ray radiation dose and the gain have a mutual relationship that an adequate image cannot be obtained unless one of the two values is increased and the other value is decreased. Consequently, it is not easy for the operator to control the two apparatuses while considering this relationship, or the operator is required to have long experience and skill.